Correl function

Hi,

I am doing some correlations on data. I have 69 rows of two columns. 

I have 3 groups (subsets) of data (28 rows, 29 rows and 12 rows). When I use CORREL on the two columns for each of these subsets I get the following results: 

But when I use the CORREL function on all 69 rows I get a result of 0.002.

Does this make sense? Can anyone explain it to me?

Thanks!!!